﻿// Copyright (c) 2012-2013 fancidev
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

using System;

namespace Util.IO.Compression
{
    /// <summary>
    /// Computes the Adler-32 checksum.
    /// </summary>
    /// <remarks>
    /// See http://en.wikipedia.org/wiki/Adler-32 for details on the algorithm.
    /// </remarks>
    public class Adler32Checksum
    {
        private int a = 1;
        private int b = 0;

        /// <summary>
        /// Updates the checksum by calculating the specified buffer range.
        /// </summary>
        /// <param name="buffer">A buffer of bytes.</param>
        /// <param name="offset">Index to start calculation.</param>
        /// <param name="count">Number of bytes to calculate.</param>
        public void Update(byte[] buffer, int offset, int count)
        {
            for (int i = offset; i < offset + count; i++)
            {
                a = (a + buffer[i]) % 65521;
                b = (b + a) % 65521;
            }
        }

        /// <summary>
        /// Gets the current checksum value.
        /// </summary>
        public int Value
        {
            get { return (b << 16) + a; }
        }
    }
}
